1. Field of the Invention
This invention relates to a process for the production of unsaturated and polyunsaturated hydrocarbons from saturated and monounsaturated hydrocarbons, respectively, and contains two essential phases: a dehydrogenation phase, wherein the hydrocarbons are adiabatically passed through a fixed catalyst bed, and a catalyst regeneration phase, wherein the catalyst is regenerated by passing an oxygen-containing gas through the fixed bed. This two-phase process is characterized by a counterflow arrangement wherein the flow direction of the oxygen-containing gas in the regeneration phase is opposite that of the hydrocarbon feedstock in the dehydrogenation phase.
2. Description of the Prior Art
It has long been recognized that coke is deposited onto the surface of the catalyst during the dehydrogenation process. As a result, active sites on the catalyst needed for the dehydrogenation reaction to proceed are blocked. To restore catalytic activity, the coke is characteristically removed from the surface of the catalyst by combusting it in the presence of an oxygen-containing gas. Accordingly, a cyclical operation is established wherein the dehydrogenation phase of the hydrocarbons alternates with the regeneration phase of the catalyst. These phases are characteristically interrupted long enough to purge the catalyst free of hydrocarbons. This purging stage is normally conducted under inert gases. Dehydrogenation processes of this type are more fully described in U.S. Pat. Nos. 3,647,909, 3,711,569, 3,781,376, 4,012,335, and 4,371,730.
Dehydrogenation of the unsaturated product is a problem often encountered in such prior art processes. In the dehydrogenation phase, the hydrocarbons are contacted with the catalyst at temperatures between 500.degree. to 650.degree. C. Dehydrogenation typically occurs within several minutes to one hour, depending on the reaction conditions. During the reaction period, the hydrocarbon conversion diminishs continuously, what is attributed to (1) a reduction in the activity of the catalyst due to the deposition of coke on the catalytic surface and (2) the cooling of the catalyst bed by the reaction itself. This latter effect is attributable to the fact that the quantity of heat withdrawn from the catalyst bed by the endothermic dehydrogenation reaction is greater than that which can be supplied by the stream of hydrocarbons.
When the amount of coke deposited on the surface of the catalyst is small, insufficient heat is generated during the regeneration phase to replace the heat consumed by the endothermic dehydrogenation reaction. As a result, the fixed catalyst bed is not completely heated to the requisite temperature for dehydrogenation to effectively proceed. To compensate for this deficiency, the oxygen-containing gas, employed in the regeneration phase, is preheated to 600.degree.-700.degree. C. and is passed through the catalyst bed longer than is necessary to merely oxidize the coke.
A technique described in the above cited U.S. patents and employed in a number of industrial installations is to add gaseous or liquid fuel equivalent to the amount of heat desired to the oxygen-containing gas before it enters the reactor. This fuel is commonly referred to as "injection fuel" and is burned in the catalyst space. Such injection fuel produces the additional heat needed to reheat the catalyst bed.
In the prior art processes, the oxygen-containing gas passes through the reactor during the regeneration phase in the same direction as the hydrocarbon feed mixture undergoing dehydrogenation. In other words, the hydrocarbon and the oxygen-containing gas enter the catalyst bed at different times but at the same location. During its passage through the catalyst bed, the regeneration gas is subsequently cooled. Since the available time for the regeneration phase is limited, a steady state is not established within the bed. A somewhat steep temperature drop develops therefore in the catalyst bed toward the end of the reactor. Since the amount of dehydrogenated hydrocarbon produced decreases at lower temperatures, such decreasing temperature profiles effect the amount of product being produced. Further, since at the beginning of the reaction phase the freshly regenerated catalyst is very active, reduced temperatures have a severe disadvantageous effect upon the yield.
The prior art processes are further disadvantageous over the present invention since they are accompanied by cracking reactions. Such reactions are attributed to heating the hydrocarbon fuel mixture at high temperatures prior to feeding them into the reactor.
German OS No. 23 04 280 discloses a sulfide recovery process employing a counterflow arrangement. In this process, a bauxite catalyst is employed in the production of elementary sulfur which, in turn, is produced by partially combusting a hydrogen sulfide stream with air. Upon the bauxite catalyst, coke-like particles are deposited. These deposits are removed from the catalyst by burning the catalyst with molecular oxygen in the presence of an inert gas. The regeneration gas is then passed in the flow direction opposite to that employed for normal operation of the sulfur recovery unit. In such sulfide recovery processes, substantial quantities of liquid sulfur residues remain in the conversion zone and are not driven off by the purge gas. They can be removed by the physical action of the stream of the regeneration materials. The conditions, purpose and effect of passing the regeneration gas stream in the counterflow direction to that of the reaction gas stream radically differ in such sulfur recovery processes compared to the dehydrogenation process of this invention.